I had a 7 x 5 trailer made up of chequer plate (very sturdy) and when i got the guy to build it I got him to manufacture a rack that bolts into the front of the trailer with two wheel channels, just to hold the front wheels in position.
on this rack he has welded some tie down points and I put an eyelet on each side of the trailer to also tie the bikes to.
it fits 2 bikes very nicely accross and length wise.
When not using it as a bike trailer I pull the rack out and I have a good box trailer.